By examining the posterior portions of a subject's eye, such as the retinal vessels and the choroidal vessels, a significant amount of data can be collected for various diagnostic and analytical applications. For example, the blood oxygen saturation or blood oxygen content of the blood in the retinal vessels is determinative of the arteriovenous oxygen difference as described by U.S. Pat. No. 5,308,919 to Thomas E. Minnich, U.S. Pat. No. 5,776,060 to Matthew H. Smith, et. al., and U.S. Pat. No. 5,935,076 to Matthew H. Smith, et. al. Based upon the arteriovenous oxygen difference, the cardiac output of the subject can be determined in order to assist in post-operative monitoring and the management of critically ill patients. By monitoring the blood oxygen saturation, the loss of blood can also be detected and the rate and quantity of blood loss over time can be estimated as described by U.S. Pat. No. 5,119,814 to Thomas E. Minnich.
In addition to blood oxygen saturation, retinal tissue perfusion can be monitored by observing the color of cytochromes, such as cytochrome oxidase a3, within the tissue cells. Further, by observing the magnitude of the choroidal reflectance pulsation across the cardiac cycle, such as with a pulse oximeter, the choroidal oxygenation can also be monitored. See J. P. de Kock, L. Tarrassenko, C. J. Glynn, A. R. Hill, "Reflectance Pulse Oximetry Measurements from the Retinal Fundus," IEEE Transactions on Biomedical Engineering, vol. 40, no. 8, 817-23 (1993). In addition to the foregoing examples, a number of other anatomic features and physiological parameters, such as the degree of melanin pigmentation of the fundus, can be determined by examining the posterior portion of a subject's eye, as described by F. C. Delori, K. P. Phlibsen, "Spectral reflectance of the human ocular fundus," Applied Optics, vol. 28, no. 6, 1061-77 (1989).
In order to examine the posterior portion of a subject's eye, several non-invasive techniques have been developed. In this regard, the traditional technique for examining the posterior portions of a subject's eye is fundus photography. Fundus photography illuminates a subject's eye with a flash of white light. Fundus photography then detects the light returning from the subject's eye, such as a result of the reflection of a portion of the light from the retinal and choroidal vessels, as well as the reflection and scattering of portions of the light from other features of the posterior portion of the subject's eye. Since the subject's eye is initially illuminated with white light, fundus photography typically spectrally separates the light that returns from the subject's eye in order to separately evaluate the signals that return from the subject's eye at different wavelengths of interest.
Because traditional fundus photography involves illuminating the subject's eye with a flash of white light, care must generally be taken to ensure that the subject's eye is not exposed to excessively high levels of light that could harm the subject's eye. In this regard, the Food and Drug Administration has defined maximum allowable amounts of light to which an eye can be exposed over a certain time period. As such, the intensity of the flash of white light must generally be maintained below some relatively low threshold, such as below about 100 mJ/cm.sup.2 in order to protect the subject's eye. Since the flash of white light must generally be maintained below some threshold, the resulting signal to noise ratio of the signals returning from the posterior portion of the subject's eye is also relatively low in comparison to the higher signal to noise ratios that would be obtained if the flash of white light could have a greater intensity. As a result of the relatively low signal to noise ratio, at least some of the signals returning from the posterior portions of the subject's eye generally are lost in the noise, and the overall validity or credibility of the signals returning from the posterior portions of the subject's eye is subject to more questions.
Even though the flash of white light that illuminates a subject's eye in traditional fundus photography is somewhat limited in order to protect the subject's eye, the flash of white light is still generally sufficiently intense to prevent closely spaced, serial measurements from being obtained. In this regard, the flash of white light is still sufficiently intense to cause the pupil of the subject's eye to constrict and, in some instances, to cause the metabolism of the subject's eye to be altered. In order to monitor the posterior portions of the subject's eye under consistent conditions, traditional fundus photography must therefore wait until the subject's pupil is no longer constricted prior to collecting another image of the posterior portions of the subject's eye, thereby disadvantageously delaying the entire examination process.
More recently, scanning laser opthalmoscopes have addressed at least some of the shortcomings of traditional fundus photography. For a general description of scanning laser opthalmoscopes, see Noninvasive Diagnostic Techniques in Ophthalmology, Barry R. Masters, editor, Chapter 22, Scanning Laser Ophthalmoscope, by Robert H. Webb, Springer-Verlag, N.Y. (1990). Conventional scanning laser opthalmoscopes have a single laser source. The scanning laser opthalmoscope scans the laser signals emitted by the laser source in a predetermined pattern across posterior portions of a subject's eye to thereby define a frame having a number of scan lines. Since a single laser is employed, the resulting image will only provide information relating to the posterior portion of the eye at the one particular wavelength.
A more recent scanning laser opthalmoscope developed by OPTOS P.L.C. of Fife, United Kingdom apparently includes three laser sources, each of which emits light of a different wavelength. This scanning laser opthalmoscope is designed to simultaneously illuminate the subject's eye by scanning laser signals emitted by each of the three lasers across the posterior portion of the subject's eye.
This scanning laser opthalmoscope also typically includes dichroic beam splitters for separating the signals that return from the posterior portions of the subject's eye based upon the wavelength of the return signals. As such, the return signals attributable to the laser signals emitted by each of the laser sources are effectively separated and can therefore be individually analyzed.
Since this more recent scanning laser opthalmoscope simultaneously illuminates a subject's eye with the laser signals emitted by each of the laser sources, this scanning laser opthalmoscope may also have the potential to expose the subject's eye to excessive amounts of light. As such, the intensity of the laser signals emitted by each of the laser sources is generally maintained at a relatively low level, such as below about 10 uW such that the cumulative intensity of the laser signals remains safe for the subject's eye. As such, the signal-to-noise ratio of the signals returning from the posterior portions of the subject's eye is therefore correspondingly reduced relative to the signal-to-noise ratio of the return signals that would be possible if the intensity of the laser signals emitted by each of the laser sources were not reduced in order to protect the subject's eye. At least some of the signals returning from the posterior portion of the subject's eye will therefore be lost in the noise and the overall validity of the return signals will be somewhat more questionable due to the lower signal-to-noise ratio.
In addition, it has been proposed that a scanning laser opthalmoscope could sequentially scan the posterior portions of a subject's eye with the laser signals emitted by the plurality of laser sources. See U.S. Pat. No. 5,815,242 to Douglas C. Anderson, et al. and assigned to OPTOS P.L.C. that describes a scanning laser opthalmoscope having first, second and third laser sources. In operation, the scanning laser opthalmoscope initially scans the posterior portions of the eye with the laser signals emitted by the first laser so as to create a first data frame based upon the return signals. Thereafter, the scanning laser opthalmoscope scans the posterior portions of the eye with laser signals emitted by the second laser source in order to create a second data frame. Finally, the scanning laser opthalmoscope scans the posterior portions of the eye with the laser signals emitted by the third laser source in order to create a third data frame. While sequentially scanning the posterior portions of the eye with the laser signals emitted by the different laser sources permits the laser sources to emit laser signals having a greater intensity, the overall examination process takes longer since separate data frames must be constructed for the laser signals emitted by each of the laser sources. In addition, since the posterior portions of the eye are scanned with the laser signals emitted by each of the different laser sources at different intervals of time, the test conditions may change between the times at which the eye is exposed to laser signals from different ones of the laser sources. For example, the subject may move slightly, thereby altering the area of the peripheral portion of the eye that is illuminated and therefore examined. Alternatively, the subject's pupil may constrict as a result of the laser illumination. Since the test conditions can change between the time at which the laser signals emitted by one of the laser sources are scanned across the posterior portions of the eye and the time at which the laser signals emitted by another one of the laser sources are scanned across the posterior portions of the eye, the consistency and correlation between the data frames attributable to the laser signals emitted by each of the laser sources are limited.
Accordingly, while several techniques have been developed for examining the posterior portions of a subject's eye, each of these techniques is somewhat limited. As such, an improved method and apparatus for examining the posterior portion of an eye is therefore desired which does not expose a subject's eye to excessive illumination. In addition, it would be desirable to examine the posterior portions of a subject's eye during specific predetermined portions of the subject's cardiac cycle since a number of the features that are being examined are at least somewhat dependent upon the phase of the cardiac cycle. Traditional fundus photography and scanning laser opthalmoscopes have not generally considered the phase of the cardiac cycle of the subject, but have, instead, obtained images or other data related to the posterior portions of the subject's eye without regard to the phase of the cardiac cycle of the subject. As such, the variations in those features of the posterior portion of the subject's eye that are dependent upon the phase of the cardiac cycle of the subject have typically not been taken into account.